1. Field of the Invention
This invention relates to fuel cells and more particularly to apparatus for recirculating a reactant gas within a fuel cell stack.
2. Description of the Prior Art
A basic fuel cell comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed therebetween in a compartment formed between the two electrodes. On the nonelectrolyte side of the anode electrode is a reactant gas chamber for carrying a fuel into contact with the anode electrode. On the nonelectrolyte side of the cathode electrode is a reactant gas chamber for carrying an oxidant into contact with the cathode electrode.
One of the reaction products of a fuel cell is water. The reactant gases passing through the cells of a stack have the job of carrying excess water from the cell stack. In a cell using a base electrolyte and 100 percent oxygen as the oxidant (as opposed to air), the oxidant gas chamber is often dead ended. In other words, oxygen is fed into the cell stack but does not leave except for occasional purging necessary to get rid of the slow accumulation of impurities which come in with the oxygen. On the other hand, it is typical that the fuel reactant gas chambers are not dead ended. The fuel travels through the cells, picking up excess water as it passes therethrough, and is recirculated (in part) through the fuel reactant gas chamber by a pump after passing through a condenser and separator to remove excess water therefrom. Fresh fuel is, of course, added to the recirculated, partially vitiated fuel in sufficient quantities to operate the stack.
As fuel cells are pushed to higher and higher power levels a problem may occur at the inlets to both the fuel and oxidant gas chambers. That is, as more and more quantities of new, dry reactant gases come into the cells they produce a local drying of the cells near the inlets of the reactant gas chambers. This is normally not a problem on the fuel side since the recirculating fuel contains a fair amount of moisture. However, dry out of the electrodes on the oxygen side, with no recirculation, is a potential problem. Even with a recirculating oxidant system, if the gas drops below the dew point before it reaches the inlet to the cells either of two things could happen: (1) water could condense from the recirculating oxidant resulting in its being too dry as it enters the cells, which would thereby defeat one of the purposes for recirculating the oxidant in the first place, or (2) the water condensing from the recirculating oxidant could be dragged back into the inlet of the cell and produce localized discontinuities in the concentration of the electrolyte (i.e., flooding), which is also detrimental to the cell.
According to the prior art an external pump should be used to recirculate the gas; and, in order to have the dew point of the recirculating oxidant at precisely that desired for proper operation of the cell, a condenser with a temperature control is required. The condenser, of course, also requires a recirculating cooling system and a heat rejection radiator. It is thus apparent that prior art approaches to recirculating a reactant gas stream are unattractive from both a complexity point of view and a cost point of view.